1. Field of the Invention
The present invention relates in general to generation of second-harmonic laser light and, more particularly, to generation of both fixed-frequency and tunable second harmonic laser light with reduced intensity noise.
2. Description of the Related Art
A second harmonic generation laser typically includes a pump laser and a non-linear material, which converts optical energy emanating from the pump laser to optical energy of a desired frequency. Laser harmonic regeneration is well known and is described, for example, in U.S. Pat. No. 5,027,361, entitled Efficient Laser Harmonic Generation Employing A Low-Loss External Optical Resonator, invented by Kozlovsky, et al; U.S. Pat. No. 5,036,220, entitled Non-Linear Optical Radiation Generator and Method of Controlling Regions of Ferroelectric Polarization Domains in Solid State Bodies, invented by Byer, et al; U.S. Pat. No. 5,355,247, entitled Method Using a Monolithic Crystalline Material for Producing Radiation by Quasi-Phase-Matching, Diffusion Bonded Monolithic Crystalline Material for Quasi-Phase-Matching, and Method for Fabricating Same, invented by Byer, et al; U.S. Pat. No. 5,644,422, entitled Techniques of Radiation Phase Matching Within Optical Crystals, invented by Bortz, et al, each of which is expressly incorporated herein by this reference.
Non-linear optical crystals have been used to double the frequency of an incident laser beam through generation of a second harmonic within the crystal. Non-linear optical crystals also have been used to generate laser radiation that has a frequency equal to the sum or difference of the frequencies of two incident radiation beams. There are many materials, referred to herein as non-linear frequency converting media, that have been used or suggested over the years for use as a mixing crystal, such as KTP (KTiOPO4), lithium tantalate (LiTaO3), and lithium niobate (LiNbO3). It is common to use such crystals as a second harmonic generator that doubles the frequency output of a pump laser source. This allows the use of long wavelength lasers, such as those in the infrared region of the spectrum, in a system that generates shorter wavelength light in the green or blue portion of the spectrum.
Intensity noise can arise in SHG laser systems primarily from the pump laser source. Traditional pump laser sources have been noisy due to modal instability, wavelength instability and broad output spectra, for example. Resulting fluctuations in pump laser output can be amplified in the non-linear optic material, resulting in SHG output intensity fluctuations that typically can vary from 1–5%.
For instance, a typical Fiber Bragg Grating stabilized laser ordinarily operates with multiple modes. As used herein, a mode designates the number of half-wavelengths within a laser cavity. Optical energy may shift between modes. In other words, the proportion of the total optical energy produced by a pump laser that is within any given mode may change as the energy shifts among the multiple modes.
Unfortunately, mode changes of the fundamental frequency produced by a laser pump source can result in a noise spike in the harmonic wavelength produced by the non-linear optical medium. In the past, Fiber Bragg Gratings (FBGs) have been used to stabilize pump lasers. However, this approach can be inefficient because the pump laser emission wavelength should match the acceptance frequency range of the non-linear optics. As a result, for example, it may be necessary to temperature-tune the non-linear optical medium to match the wavelength of the pump laser. Also, due to the relatively long cavity length in an FBG stabilized pump laser, several optical modes may coexist within the cavity, thereby increasing the intensity noise of the laser.
Thus, there has been a need for an improved system for laser harmonic generation with reduced intensity noise. The present invention meets this need.